1. Field of the Invention
The present invention relates to air intake filters for internal combustion engines and other air aspirating machines and, more particularly, to a disposable dual-element filter cartridge which is designed for use in air intake filters of air aspirating machines operating in a dusty environment.
2. Description of the Prior Art
Air aspirating machines which have to operate in a dusty environment include, for example, internal combustion machines of commercial vehicles, farm machinery and construction machinery, as well as air compressors and the like. The air intake filters of these machines are expected to have disposable filter cartridges with a large dust accumulation capacity, for a correspondingly long service live, and they have to operate with a good filtering efficiency over the entire duration of their service live.
An additional requirement, common to all air intake filters, is a low flow resistance across the filter. This flow resistance increases gradually, as the layer of accumulated dust on the filter element grows thicker, until it reaches a predetermined critical flow resistance at which the dirty filter cartridge has to be replaced.
For air intake filters of internal combustion engines, the most widely used type of filter cartridge is one which has an annular filter element of pleated filtering paper which is traversed by the air flow in a radially inwardly directed converging air flow. This configuration of the filter element makes is possible to accommodate a relatively large effective filtering surface in a small space.
In filter applications which are designed for a dusty environment, it has become common practice to use filtering papers of between 0.3 and 0.5 mm gauge and a density between 0.2 and 0.3 g/cm.sup.3. Normally, the size of the effective filtering surface ranges between 4000 and 5000 cm.sup.2 for each m.sup.3 /min of aspirated air.
This compares to filter applications designed for a dust-free environment, such as, for example, air intake filters of automobile engines--which are assumed to be operated primarily on dust-free roadways--for which an effective filtering surface in the order of 1500 cm.sup.2 per m.sup.3 /min of aspirated air is normally chosen.
In order to accommodate in a given filter cartridge a larger effective surface of filtering paper, it has already been suggested, in U.S. Pat. No. 3,397,793, to use a filter cartridge which has two filter elements of pleated filtering paper arranged in a concentric relationship. However, this particular filter configuration is not suggested for an air intake filter, but for an oil filter which is designed for use in the engine lubrication system of a diesel locomotive. The two filter elements are shown to have the same radial depth, the inner diameter of the outer filter element being approximately twice as large as the inner diameter of the inner filter element.
According to the patent disclosure, this prior art dual-element filter cartridge makes it possible to accommodate in the same space a 25% greater effective filtering area, while offering the additional possibility of selecting a smaller pore size for the inner filter element.
In the case of air intake filters which are designed for use with internal combustion engines operating in a dusty environment, the objective of achieving an acceptable service life makes it desirable to use filtering papers of lesser density, i.e. larger flow passages or pores, in order to delay the plugging up of the pores. However, this result is being achieved at the expense of the filtering efficiency in the new state of the filter cartridge, which is lowered, as more small dust particles are allowed to pass through the larger pores of the filtering paper.
With the passage of operating time, this initially unsatisfactory filtering efficiency improves, as a layer of dust accumulates on the surface of the filter element and the flow passages or pores in the filtering paper become gradually more restricted so as to trap the dust particles which previously passed through the filter element.
It follows that, while all air intake filters are subject to the problem of the countervailing objectives of an extended service life and a satisfactory filtering efficiency in the new state, this problem is more severe in the case of air intake filters which are designed for operation in a dusty environment.
A dual-element filter cartridge of the type suggested for the earlier-mentioned oil filter, where a filter element of fine-pore filtering paper is arranged downstream of a filter element of large-pore filtering paper, would not solve this problem, because the smaller flow passages or pores of the denser filtering paper of the downstream filter element would quickly become plugged up by the dust particles which pass through the larger flow passages or pores in the filtering paper of the upstream filter element, thus actually shortening the service life of the filter cartridge.
U.S. Pat. No. 4,304,580 and the related U.S. Pat. No. 4,314,832 disclose a filter cartridge assembly in which two separate filter cartridges are arranged in a concentric configuration. The inner filter cartridge consists of a filter element of pleated filtering paper of relatively narrow radial width which is enclosed between compression-resistant inner and outer supporting sleeves of perforated sheet metal. The outer filter cartridge is designed as a disposable filter element of pleated filtering paper, having paper, having a much larger radial width than the inner filter cartridge and lacking a supporting member on its inner or outer periphery.
The inner filter cartridge of this known filter assembly serves to counteract the axial clamping pressure generated by the filter housing, while at the same time serving as a backup filter cartridge, in the event that the unprotected principal outer filter cartridge should suffer major structural damage.
Since only the outer cartridge is designed as a disposable filter cartridge which is subject to replacement when caked with accumulated dust, this arrangement does not offer any contribution in terms of a solution to the problem of the air intake filters for dusty environment, where the countervailing objectives of service life and filtering efficiency in the new state have heretofore required an unsatisfactory compromise.